1. Field of the Invention
The invention relates to a sheet supplying technique in printing apparatus which moves print head and executes a print, such as serial dot printer, an ink jet printer and the like.
2. Related Background Art
In conventional printing apparatus, as shown by FIGS. 20 and 21, a sheet supplying unit is furnished for supplying print medium 13 (at left side in FIG. 20) such as sheet to printing section P1. When the print medium 13 supplied to the printing section P1 by the sheet supplying unit skews, and arrives at the printing section P1 in the skewing state, a skewing print will be caused. Therefore, in order to prevent the print from skewing, a plurality of slip rollers 11 for rectifying such skew are arranged rotating-freely.
Further, in the conventional printing apparatus, in order to detect the skew, as shown by FIG. 21, under table 16 at the upstream side of feed roller 12 along a conveyance direction, a plurality of table sensors 14 are arranged; under table 16 at the downstream side of feed roller 12 along a conveyance direction, a plurality of skew sensors 15 are arranged.
Then, in the conventional printing apparatus, as shown by FIG. 21, after operator puts the print medium 13 onto the table 16 so as to make the print medium 13 covers at least one of plural roller substance section 11b to construct the slip roller 11, through the table sensor 14 detected that the print medium 13 has been put, a skew rectifying motor (not shown in figure) is driven, the slip roller 11 is rotated and the print medium 13 is conveyed.
With the conveyance of the print medium 13, the front edge (that is, in FIG. 21, it is upper edge of the sheet) of the print medium 13 touches with a contact point of a predetermined roller substance section 12b′ in respective roller substance section 12b to construct the feed roller 12.
At that time, because the roller substance sections 11b and the roller substance sections 12b are respectively arranged in corresponding positions each other, while the front edge of the print medium 13 touches with a contact point of a predetermined roller substance section 12b′, the roller substance section 11b′ being in the roller substance sections 11b and corresponding to the predetermined roller substance section 12b′ is made to slip with respect to the print medium 13, the conveyance of print medium 13 through the roller substance section 11b′ is not executed.
Because the other roller substance sections 11b continue to execute the conveyance of the print medium 13, the front edge of the print medium 13 sequentially touch with respective contact points of each of roller substance sections 12b, then the respective roller substance sections 11b corresponding to the respective roller substance sections 12b are respectively made to slip with respect to the print medium 13, the conveyance of the print medium 13 through the respective roller substance sections 11b are not executed. That is, the print medium 13 is rotated to the position of print medium 13′ along a direction A in FIG. 21.
Thus, the skew which occurred when putting the print medium 13 is rectified. Moreover, the roller substance section 11b is made from soft rubber material so as to possibly sufficiently slip with respect to the print medium 13.
Then, after the slip roller 11 only rotated a predetermined quantity, through driving a line feed motor, the feed roller 12 is rotated and the print medium 13 is conveyed to the printing section P1.
Further, after the front edge of the print medium 13 arrived at the skew sensors 15 while conveying the print medium 13, the skew sensors 15 detects a detection difference of left and right of the print medium 13. If the detection difference exceeds a threshold value, it is judged that the skew is not rectified, the line feed motor is driven to rotate along a contrary direction, and the print medium 13 is discharged.
Then, if the detection difference is under the threshold value, it is judged that the skew has been rectified, the line feed motor continues to be driven. As a result, the feed roller 12 is executed to rotate along a supplying-sheet direction, the print medium 13 is supplied to the printing section P1, and a print is executed in the printing section P1. (for example, it may refer to patent document 1)    Patent document 1: Japanese patent publication 2002-193492.
However, in the sheet supplying unit of the conventional printing apparatus, it is necessary to arrange a great many skew sensors 15 along the print medium width direction, the structure of the printing apparatus becomes complex, and the cost of the printing apparatus becomes high.